feat: switch to biome, apply formatting, shellcheck (#227)

* feat: switch to biome, apply formatting, shellcheck
* chore: apply cr comments
* chore: few config tweaks, shellcheck hook now py-based
* chore: lint fixes and pr comments
* chore(lint): megalinter, and other fixes

Signed-off-by: Ismo Vuorinen <ismo@ivuorinen.net>

local/bin/x-thumbgen

@@ -126,8 +126,8 @@ parse_options()
 # Generate thumbnails recursively using find and filtering by MIME type
 generate_thumbnails()
 {
-  local source_dir=$1
-  local output_dir=$2
+  local source_dir="$1"
+  local output_dir="$2"
 
   # Ensure the output directory exists (create if necessary)
   if [ ! -d "$output_dir" ]; then
@@ -144,7 +144,7 @@ generate_thumbnails()
     fi
 
     # Determine the relative path with respect to the source directory.
-    rel_path="${file#$source_dir/}"
+    rel_path="${file#"$source_dir"/}"
     dir="$(dirname "$rel_path")"
     base="$(basename "$rel_path")"
     filename="${base%.*}"